Put the flour, sugar, and salt in a food processor and pulse to combine. Drop in the pieces of butter and pulse the mixture until it looks like coarse meal, with bigger clumps of butter the size of a pea. With the machine running, pour in the cold water gradually, adding just enough to allow the dough to start pulling away from the side of the bowl; if you squeeze it together, it will form a dough. Dump out onto a piece of plastic wrap, pat it down to flatten into a disc, then wrap it in plastic and chill it for about 1 hour, or up to 3 days. (It also freezes for up to 2 months.)
When you’re ready to bake, preheat the oven to 350°F. Line a baking sheet with parchment and put the dough between two sheets of parchment. If it's very firm, you may need to bash it a bit with your rolling pin to soften it up. Roll it into a rectangle that’s about 5x11-inches. Transfer the dough to the lined baking sheet and peel off the top piece of parchment. Beat the egg yolk with a few drops of cold water and brush the surface all over. With the tines of a fork, decorate the top with a crosshatch pattern.
Bake for 30-35 minutes, or until golden. It will be firm to the touch but will have a little spring when pressed in the center — the perfect break-up is crisp on the outside and still tender within. Transfer to a wire rack to cool. Serve it whole on a platter or cutting board, set in the middle of the table.